The under-fire Red Devils boss had felt Sunday's point at rivals Leeds United marked a "fresh start" after defender Lisandro Martinez had articulated the club's shortcomings in a set of stunning interviews last week.
But United instead saw their wretched start to the season hit a new low on Wednesday (Thursday AEDT) as the Serie A side built by former Spanish star Cesc Fabregas punished a display devoid of shape, grit and skill to extend their winless run to seven matches.
"First of all, I have to apologise to the supporters" United boss Carrick said.
"For us to play in that way is not acceptable. It wasn't a good performance at all ... I have to say. It was tough to watch."
It was also a night to forget for Socceroos pair Nestory Irankunda and Giannis Stensness.
Irankunda started and played 79 minutes in Sporting Lisbon's shock 3-2 home loss to LASK, who were beaten 6-3 in Austria by Liverpool last week.
And seven days after scoring in a thrilling 3-3 draw with Bayern Munich, central defender Stensness played a full game in Viking's 3-1 defeat at Aston Villa.
The Birmingham club maintained their perfect start to the campaign and climbed to second in the standings after three matches.
Ross Barkley nodded Villa in front against the Norwegians, Emi Buendia doubled their advantage from the spot and Henrik Heggheim's own goal made the result safe.
In Munich, Declan Rice stole the limelight from Harry Kane as Arsenal snatched a thrilling 2-2 draw against Bayern in a heavyweight clash.
Gunners midfielder Rice lashed low beyond goalkeeper Manuel Neuer eight minutes from time to extend his side's unbeaten start to this season's competition following wins over Napoli and Lille.
Kane – Tottenham's all-time record scorer – was on course to claim the winner after registering his 16th goal in 23 career outings against Arsenal by converting a 61st-minute penalty.
The England captain earlier produced a sublime assist for Alphonso Davies to cancel out Eberechi Eze's opener.
In Italy, United made a nightmare start, just as they did at Atletico Madrid last week, as Nico Paz struck two minutes in.
A 49th-minute Tasos Douvikas strike, followed by Paz's second and Jacobo Ramon heading home made it 4-0.
Joshua Zirkzee added a consolation goal on a night that ended with Carrick apologising in front of United fans that stayed until the end.
Elsewhere, Real Madrid thumped RB Leipzig 4-1, Galatasaray triumphed 4-2 at Lille, Club Brugge beat Lens 3-0 and both Inter Milan and Real Betis won 1-0.
With AAP.
